KVM: x86: Convert kvm_arch_vcpu_reset into private kvm_vcpu_reset

There are no external callers of this function as there is no concept of
resetting a vcpu from generic code.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>